There any many ways of making gold. Iam a miner and a smithy so I dont sell my mined ores.
I basically go the ah rought and find rare items going for cheap (compare the item on
http://www.wowhead.com/) if its going for cheap buy it and sell it for the suggested price, 90% of the time i get my price. I put my start price and buyout price close together (eg: start 90g, buyout 95g). Always check the trade channel you can get good deals there as the ppl there are in a hurry (most of the time) to get the g for the item.
Another way I go is finding out where rare spawns, spawn and I go there once a week. I only farm those who drop rare items. eg. there is a rare spawn in WC just before you enter the instance in the lake. He always drops a rare item (quick way of making 25g for 5 min work. There are many other rare spawns check
www.thottbot.com under rare items and see who drops them.
Grind the graveyard in SM. quick way of getting some good greens and a blue (maybe) & loads of silver. The NPC's there are not elite.
Grind the Whelpings in the Dustwallow Marsh and Badlands, if you lucky you will pickup a Dark Whelping (
http://wow.allakhazam.com/db/item.html?witem=10822) I put this little guy in the ah, advertised on the trade channel (start 90g buyout 100g) not even a minute later it was sold.